Passive splitter filter for digital subscriber line voice communication for complex impedance terminations

ABSTRACT

A dual-mode filter network for an Asymmetric Digital Subscriber Line (ADSL) has a first frequency response when the Plain Old Telephone Service (POTS) telephone is on-hook and has a second frequency response when the POTS telephone is off-hook. A detector is used to determine the on-hook/off-hook status. The dual-mode filter network may be situated at the central unit and connected to a Public Switched Telephone Network (PSTN) while the detector at the remote unit connected between the POTS low pass filter and the POTS telephone. In such case, the detector outputs a signal which is transmitter via the twisted pair on an overhead channel. Alternatively, the dual-mode filter and the detector may be co-located at either the central unit or at the remote unit. Regardless of where it, or the detector are located, the dual-mode filter network has at least different combinations of components which are activated, depending on the on-hook/off-hook status of the POTS phone. In addition, a single-mode filter instead of the dual-mode filter can be used at the remote unit.

RELATED APPLICATIONS

This application is related to U.S. application Ser. No. 09/038,938filed Mar. 12, 1998 and U.S. application Ser. No. 09/083,162 filed May22, 1998.

TECHNICAL FIELD

This invention concerns technology which facilitates simultaneous dataand voice traffic over a communication channel. More particularly, itrelates to devices arranged to filter voice signals sent across standardtwisted pair telephone lines.

BACKGROUND OF THE INVENTION

Asymmetric Digital Subscriber Line (ADSL) is a technology which allowsfor simultaneous voice and data traffic to coexist over a communicationchannel comprising a standard telephone transmission line. Typically,the standard telephone transmission lines comprise an unshielded twistedpair of copper wire having a gauge of 22-26AWG. Twisted pairs, which canbe used to connect a central telephone system (a `central` unit) to asubscribers telephone (a `remote` unit) can support bandwidths of up to2 MHz through the use of digital signal processing (DSP) technology.Thus, they can be used for bandwidth-intensive applications, such asinternet access and video-on demand, as well as for carrying voicetraffic. Frequency division multiplexing is used so that a plurality ofsignals, each occupying a different frequency band, can besimultaneously sent over the same transmission line.

The voice traffic band comprises a number of frequency sub-bands, orchannels, ranging from DC to 20 KHz. The analog voiceband frequency istypically specified as 200-4000 Hz. Customer specified additions mayinclude phone operation up to 8 KHz and 12-16 KHz billing tones. Inaddition, DC to 30 Hz frequencies are typically assigned for auxiliaryanalog signaling purposes, such as ringing the telephone, dial pulsingand on/off hook signaling.

ADSL data traffic bandwidth for CAP (carrierless amplitude and phase)modulation is typically from 35 KHz-1.5 MHz. Of this, upstream datatraffic (i.e., remote unit to central unit) uses the 35 KHz-191 KHzband, while the downstream traffic (i.e., central unit to remote unit)uses the 240 KHz-1.5 MHz band. The bandwidth for DMT (discretemulti-tone modulation) starts at 25 KHz.

As both data traffic and voice traffic are sent over the same physicalchannel, the differing types of signal traffic being received over thetransmission line must be distinguished from one another at both theremote unit and the central unit. In addition, voice and data signals tobe transmitted over the transmission line must be properly combined ateach end. In prior art systems, Plain Old Telephone Service (POTS)separation filters, installed at both the remote unit and at the centralunit are used for this purpose.

FIG. 1 shows the typical arrangement of an existing system for handlingvoice and data traffic over a physical channel comprising a transmissionline 100 comprising a twisted pair of wires 104, 106. The existingsystems must operate when the phone 118 is either on-hook (POTS signalsmay include ringing signals and/or on-hook transmissions such ascaller-ID) or off-hook (POTS signals may include tone dialing, pulsedialing and voice).

At the remote end 110, the transmission line is connected to a remote'shigh pass filter (HPF) 112 and a remote's low pass filter (LPF) 114, thefilters 112, 114 being arranged in parallel. The output of the remoteHPF 112 is then sent to a remote ADSL transceiver 116 which connects toadditional data links in a known manner. The output of the remote LPF114 connects to a telephone 118 or answering machine, or like, in aknown manner.

At the central unit 120, the transmission line 100 is connected to acentral unit high pass filter (HPF) 122 and a central unit low passfilter (LPF) 124, the filters 122, 124 again being arranged in parallel.The output of the central unit HPF 122 is presented to the central unitADSL transceiver 126, from which it can connect to additional data linksin a known manner. The output of the central unit LPF 124 is presentedto a public switched telephone network (PSTN) 128 for connection toother subscribers at other remote systems, long distance services, andthe like.

The filters 112, 114, 122 and 124 must meet certain performancecriteria. In the ADSL frequency range, the LPF 114, 124 input impedancemust be high enough not to load down the transceiver input, whichgenerally has a resistance of 100 Ω. On the other hand, in the voicebandfrequency range, the HPFs 112, 122, must have a high enough impedance soas not to load down the telephone 118.

In addition to impedance criteria, the various filters must also meetcertain performance specifications. For instance, the LPF filters mustmeet stopband criteria to prevent POTS signaling from causing errors onthe ADSL line. POTS signaling which can create errors include ringingsignals (20 Hz), broadband ringing transients caused by central unitrelays that apply and remove the ringing signal, on-hook/off-hooktransients created by a subscriber picking up a handset to make a call,and a ring trip transient caused by a subscriber at a remote telephoneanswering an incoming call, among others.

In addition to stopband criteria, the LPF filters must also meetpassband (200 Hz-4000 Hz) criteria. These passband criteria includeinsertion loss (at 1000 Hz), passband ripple, return loss (measure ofhow close the input impedance matches the off-hook load), envelope delaydistortion and longitudinal balance, among others.

In telephone networks which have off-hook termination impedances thatare purely real (i.e., no imaginary component) the task of meeting boththe passband and stopband performance specifications can be achieved byusing only passive filters.

However, in telephone networks where such impedances are complex and notpurely real, achieving the passband and stopband performance criteriausing only passive filters is very difficult. This is because of thewide variety of potential POTS signals which must be handled by thefiltering system. This difficulty is shown in the IEEE article writtenby John Cook and Phil Sheppard, "ADSL and VADSL Splitter Design andTelephony Performance" (IEEE Journal On Selected Areas inCommunications, Volume 13, Number 9, December 1995). In this article, anexample passive filter is shown. This was an 8th order modified ellipticlow pass filter with a cutoff frequency of 42.5KHz. The pole was pushedout to 42.5KHz in order to increase the passband return loss performanceto 12 dB at 4KHz. The point of the exercise was to show that having thepole at 42.5 KHz was unacceptable since this collided with the ADSL databandwidth. In addition, even with the pole at 42.5KHz, the 12 dB returnloss number was still unacceptable since the specification is 18 dBminimum.

Due to the difficulty of meeting the criteria with only passive filters,active filters are used to make the complex load look real through theuse of impedance converters.

U.S. Pat. No. 5,623,543 to Cook is an example of an active filteringapproach used to accommodate the various POTS signals. However, such adesign adds complexity, cost, power consumption, and board real estateover the traditional passive design.

U.S. Pat. No. 5,627,501 to Biran et al. is an example of a passivefilter approach. This design calls for a pair of low pass filtersconnected in series between the transmission line and the POTS receiverat the remote unit. One of the two low pass filters is always activated,while the second is selectively activated by a control signal created atthe remote unit. The control signal detects the attenuation caused bythe transmission line, due to the latter's length, and activates (ordeactivates) the second filter accordingly. While this design is able tolimit current flow through the lowpass filter to prevent saturation ofthat filter, it requires monitoring of transmission line loss. Moresignificantly, this design does not take into consideration filterperformance in both on-hook and off-hook conditions.

SUMMARY OF THE INVENTION

One objective is to provide an apparatus which achieves the requisitestopband and passband performance specifications under both on-hook andoff-hook conditions, in ADSL systems in which the central and remoteunits have complex impedances.

Another objective of the invention is to provide such an apparatus whichhas lower cost, reduced power requirements, and takes up less boardspace, than existing active filtering arrangements.

These and other objectives are realized by a system in accordance withthe present invention. Such a system includes a dual-mode filter networkwhich can be operated in a first mode when the POTS unit is on-hook andin a second mode when the POTS unit is off-hook. The filter network canbe positioned at either the central unit, between the transmission lineand the PSTN, or at the remote unit, between the transmission line andthe POTS. In either situation, the filter network is controlled by acontrol signal reflective of whether the POTS telephone is on-hook oroff-hook. The control signal is provided by a detector positioned eitherat the remote unit or the central unit.

A further aspect of the present invention is a filter network comprisingdiscrete elements. The filter network can be configured in a number ofways. One configuration is a pair of filters connected in series,wherein only one filter is active during a first mode, and both filtersare active during a second mode, thereby forming a linear system ofcascaded filters. A second configuration is a pair of filters connectedin parallel, wherein only one filter is active during a first mode andonly the other filter is active during a second mode. A thirdconfiguration for the filter network is a hybrid system wherein certaindiscrete elements are activated or deactivated when one switches fromthe first mode to the second mode.

A further aspect of the present invention is that when the detector andthe filter network are not located at the same unit, the control signalis sent across the transmission line via an overhead channel.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention can better be understood through the attachedfigures in which:

FIG. 1 shows a block diagram of a prior art asymmetric digitalsubscriber line (ADSL) system;

FIG. 2 shows a block diagram of an ADSL system in accordance with thepresent invention;

FIGS. 3A-3C show alternate configurations for a filter network to beused in the embodiment of FIG. 2;

FIG. 4 shows a circuit diagram for a detector used in the presentinvention;

FIG. 5 shows a preferred embodiment of a dual-mode filter network forthe circuit of FIG. 2;

FIG. 6 shows a first preferred embodiment of a single-mode filternetwork for passband shaping;

FIG. 7 shows a second preferred embodiment of the single-mode filternetwork for passband shaping; and

FIG. 8 shows a third preferred embodiment of the single-mode filternetwork for passband shaping.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Aforementioned U.S. Pat. Nos. 5,623,543 and 5,627,510 are incorporatedby reference to the extent necessary to understand the presentinvention.

FIG. 2 shows a block diagram of a preferred embodiment of a system inaccordance with the present invention. A twisted pair 202 connects acentral unit 204 with a remote unit 206. The central unit 204 isprovided with an ADSL transceiver 210 which sends and receives data atthe central unit 204 via an ASIC-implemented field programmable gatearray 212 under the control of a processor 214. Similarly, the remoteunit 206 has a corresponding ADSL transceiver 220 which sends andreceives data at the remote unit 206 via an ASIC-implemented fieldprogrammable gate array 222 under the control of a processor 224.

In the preferred embodiment, ADSL transceivers 210, 220 comprises astandard ADSL chip set of CAP modulation transceivers, such as thoseavailable from Globespan; field programmable gate arrays 212, 222 can beimplemented as part no. XC4020 available from Xilinx, and processors214, 224 can be implemented part no. 80C188 from Intel. Theconfiguration described thus far is well known in the prior art.

The present invention further includes a dual-mode filter network 230located at the central unit. Filter network 230 filters voicebandsignals between the twisted pair 202 and the PSTN 232. The filternetwork 230 filters the voiceband signals in one of two modes, based ona control signal 234 from the gate array 212. The control signal 234drives a relay 236 which activates connections within the filter network230, selectively causing the filter network 230 to operate in either afirst or a second mode. In the preferred embodiment, the relay 236 isimplemented as a standard, continuous contact, double pole double throwrelay.

At the remote unit 206, the voiceband signals from the twisted pair 202pass through a low pass filter 240 before being presented to the POTSphone 242. Low pass filter 240 is a standard filter of the sort commonlyused at remote units in the prior art, and is similar to that shown inFIG. 6 of aforementioned U.S. Pat. No. 5,623,543 to Cook.

The control signal 234 is reflective of whether the POTS phone 242 atthe remote unit 206 is on-hook or off-hook. Control signal 234 is logichigh (+5 V) when the POTS phone 242 is on-hook and is logic low (0 V)when the POTS phone 242 is off-hook. At the remote unit 206, a detector244 determines whether or not the POTS phone 242 is off-hook or on-hook.The detector 244 monitors the voiceband signals between the remoteunit's low pass filter 240 and the POTS phone 242 to make thisdetermination. The detector 244 then outputs a detector signal 246reflective of whether the POTS phone 242 is off-hook or on-hook. Thisdetector signal is presented to the remote unit's gate array 222 whereit is properly formatted and then sent by the remote unit's ADSLtransceiver 220 via an overhead channel across the twisted pair. At thecentral unit 204, the received, formatted signal is converted into thecontrol signal 234 in a known manner.

The preferred embodiment of FIG. 2 shows the filter network to belocated at the central unit 204 while the detector 244 is located at theremote unit 206. It should be noted however, that one may gain certainadvantages by co-locating these two. For instance, both may be locatedat the central unit, as shown in phantom by detector 248. Such analternative arrangement may be advantageous, as it would not requireretrofitting of equipment already deployed at remote units, and allinvention-related equipment could be added only at the central unit 204.Alternatively, both the filter network and the detector may be locatedat the remote unit 206. This may be useful in the planning andimplementation of future ADSL systems. Finally, if one wished, one couldeven locate the detector at the central unit and the filter network atthe remote unit. Such an arrangement may be useful for diagnosis andcontrol, by the central unit, of the remote unit's POTS performance.

The filter network 230 is designed with reference to the two modes inwhich the POTS telephone 242 must operate: on-hook and off-hook.

In the off-hook mode, voice traffic must be transmitted and so thepassband performance is of importance to ensure clarity. However, thestopband performance is not as critical, because broadband transientsbased on a ringing signal/ring-trip do not occur in the off-hook mode.Therefore, when in the off-hook mode, the ADSL system of the presentinvention requires a first set of filter performance specifications. Thefirst set of filter performance specifications is optimized for passbandperformance and has modest stopband performance sufficient to attenuatedial pulsing and off-hook-to-on-hook transients.

In the on-hook mode, no voice data is being transmitted and the passbandperformance is not as critical. However, the stopband attenuationperformance is important because of the possibility of ringingsignal/ring-trip transients. Therefore, when in the on-hook mode, asecond set of filter performance specifications are required. Thissecond set of performance specifications is optimized for stopbandperformance, but has a passband performance which is not as good as inthe off-hook mode.

FIG. 3A illustrates one embodiment which realizes the first and secondsets of filter performance specifications with a filter network inaccordance with the present invention. This embodiment provides a filternetwork comprising two mutually exclusive filters F1 and F2, each filterhaving an invariant, predetermined frequency response. Only one filteris activated at any given time, under the control of a relay RY1 whichreceives an input from a control signal. In such case, filters F1 and F2are connected in parallel and each filter has a port selectivelyconnected to the transmission line while the other port of each filteris connected to either the PSTN (if the filters are at the central unit)or to the POTS (if the filters are at the remote unit).

FIG. 3B illustrates another embodiment which realizes to realizing thefirst and second sets of filter performance specifications. Thisembodiment provides a filter network comprising two filters F1 and F2connected in series. Filter F1 has a port connected to the transmissionline and filter F2 has a port connected to either the POTS or PSTN. Whenthe filter network is operating in the first mode, only filter F1 isactive, and when it is operating in the second mode, both filter F1 andfilter F2 are active. A relay RY1 is used selectively send the signalsthrough filter F2 or through a bypass 250.

FIG. 3C presents yet another embodiment which realizes the first andsecond sets of filter performance specifications. The basic concept isthat certain discrete elements are active in both modes of operation("unaffected discrete elements") while other discrete elements areselectively activated or deactivated, depending on the mode of operation("affected discrete elements"). The affected discrete elements may beselectively connected or disconnected to the unaffected discreteelements in series, or in parallel, or via a hybrid connection, underthe control of a relay or other switch. FIG. 3C shows an example inwhich affected discrete elements A1, A2 are alternatingly activated byrelay RY1, depending upon the mode, while unaffected discrete element A3remains active, regardless of the mode of operation of the filternetwork 230. It should be kept in mind that the example of FIG. 3C showsthe aforementioned affected and unaffected discrete elements in thecontext of a more complex filter whose complete characteristics are notshown. It should also be kept in mind that this third approach alsocontemplates multiple nodes controlled by multiple relays, all inresponse to a control signal reflective of the on-hook/off-hook state ofthe POTS phone. This allows one to add, delete and shift the poles ofthe filter network in a predictable manner.

FIG. 4 shows a circuit diagram of the off-hook detector 242 of FIG. 2.When the POTS phone 242 is in the on-hook state, the voltage across thePOTS terminals 260, 262 will approach the office battery voltage ofapproximately 48 V. In this state, rectifier CR1 will cause capacitor C1to start charging through resistor R1 and capacitor C2 will chargethrough resistors R1 and R2. Eventually C2 will charge to a voltage of18 volts, which is the breakdown voltage of Zener diode D1. At thispoint, transistors Q1 and Q2, with associated biasing resistors R4 andR3, respectively, turn on, drawing the stored charge in C1. This drawscurrent through resistor R5, turning on the diode portion of U1, whichis a 4N46 opto-isolator. This turns on U1's transistor portion which hasits sensitivity dampened by resistor R6, and results in acollector-to-emitter current within U1's transistor portion. Thiscurrent causes a voltage drop across resistor R7, driving the detectorsignal line 246 low, when in the on-hook mode. When the charge across C1is depleted, the diode portion of U1 turns off, ultimately allowing thedetector signal to return to a high state. This process will repeatwhile the POTS phone 242 is on-hook, thereby creating a pulsed signal onthe detector signal line 246.

When the phone is in the off-hook state, the voltage across POTSterminals 260, 262 will never reach 18 volts. Thus, opto-isolator U1never turns on and the detector signal line 246 remains high and doesnot pulse.

The off-hook detector 244 has a real impedance of greater than 500 KΩ,and does not require series-connected elements to detect current whenthe phone is off-hook. Within the detector 244, opto-isolator U1provides telephone network voltage (TNV) isolation from the non-TNVcircuits to which the detector signal line 246 is connected; this is asafety requirement. Also, in this implementation, the SCR formed by Q1and Q2, and the charge storage provided by C1 enables sufficient currentthrough U1 without loading down the voltage across POTS terminals 260,262.

In the circuit of FIG. 2, the detector signal is qualified andintegrated within logic circuitry at the remote unit. The qualificationand integration prevents false switching between the two filter modesdue to events such as current interruptions between pulses during dialpulsing, among others. The detector signal is protected by a CRC(cyclical redundancy code) code and then sent from the remote unit, viaan overhead channel across the twisted pair. At the central unit, thedetector signal is CRC decoded and then qualified to ensure itsintegrity.

While the detector 244 shown in FIG. 4 is especially designed to bepositioned at the remote unit, one of ordinary skill in the art shouldrecognize that such a design may be adapted for use at the central unit,as well, without comprising its desirable attributes of meeting DCresistance specifications without adversely affecting passbandperformance. Table 1 lists the values for discrete elements used in thedetector circuit 244 of FIG. 4.

                  TABLE 1                                                         ______________________________________                                        Component Values for Detector Circuit FIG. 4                                  COMPONENT      VALUE                                                          ______________________________________                                        R1                       499    KΩ                                      R2                       100    KΩ                                      R3                       30.1   KΩ                                      R4                       4.02   KΩ                                      R5                       2      KΩ                                      R6                       100    KΩ                                      R7                       10     KΩ                                      C1                       0.22   μF                                         C2                       0.1    μF                                         ______________________________________                                    

FIG. 5 shows a circuit diagram 270 of a preferred embodiment of thefilter network 230 of FIG. 2. The circuit diagram 270 is animplementation of the above-described third approach, in which only asmall number of discrete elements are affected when the filter networkis switched between the first mode and the second mode. In circuit 270,the impact of switching from the first mode to the second mode, or viceversa, is manifested by contacts X1, X2 and X3. It should be understood,however, that contacts X1, X2 and X3 are controlled by a relay, whichpreferably is a make-before-break type, in order to allow for a smooth,error-free transition between modes. A relay such as TXD-2-2M-5V,available from NEC, is suitable for this purpose.

The circuit 270 comprises inductors L1, L2 and L3. Inductor L1 is acommon mode choke which is used to prevent longitudinal signalingtransients from causing ADSL data errors. Capacitor C11 and inductor L2form a first filter stage and capacitor C14 and inductor L3 form asecond filter stage. A snubber circuit 272, connected acrosscomplementary terminals of inductor L1, suppresses voltage peaks passingthrough the circuit 270. As shown in circuit 270, snubber circuit 272 isimplemented with resistor R16, capacitor C16 and Zener diodes D4 and D5.It should be kept in mind, however, that alternate designs for thesnubber circuit may work as well. The discrete elements mentioned thusfar, are always active, regardless of the mode of operation of thecircuit 270.

When the POTS phone is moved from off-hook to on-hook, the on-hookstatus is ultimately translated into a control signal which activatesrelay 236 (See FIG. 2). In this instance, the relay causes contacts X1and X2 to open and contact X3 to close. In the on-hook mode, theadditional pole circuitry 274 and 276 is not activated, resistor R11 isshort circuited and capacitors C14 and C15 are in electrical parallel.In such case, C15 and L2 provide extra stopband attenuation to suppressringing and ringing transient errors on the DSL.

When the POTS phone is moved from on-hook to off-hook, the off-hookstatus is ultimately translated into a control signal which deactivatesrelay 236. This time, the relay causes contacts X1 and X2 to close andcontact X3 to open. Under such conditions, resistor R11 keeps capacitorC5 charged to the office battery voltage. This allows for an error-freesubsequent transition from off-hook to on-hook. With X1 and X2 closed,additional pole circuitry 274, 276 becomes activated. Resistors R12,R13, R14, R15 and capacitors C12 and C13 are used in the off-hook modeto compensate for the complex load characteristics so as to optimizevoiceband performance. Thus, in the circuit 270, a number of discreteelements are activated, or deactivated, depending on the mode ofoperation, while the remainder are used regardless of the mode. Table 2lists the values for the discrete elements used in the circuit 270 ofFIG. 5.

                  TABLE 2                                                         ______________________________________                                        Component Values for Filter Circuit FIG. 5                                    COMPONENT      VALUE                                                          ______________________________________                                        L1                       5-50   mH                                            L2                       35     mH                                            L3                       9.5    mH                                            R11                      200    KΩ                                      R12                      50     Ω                                       R13                      35     Ω                                       R14                      35     Ω                                       R15                      50     Ω                                       R16                      50     KΩ                                      C11                      20     nF                                            C12                      0.47   μF                                         C13                      0.47   μF                                         C14                      20     nF                                            C15                      82     nF                                            C16                      100    nF                                            ______________________________________                                    

FIGS. 6-8 show preferred embodiments of RLC-based single-mode low-passfilter circuits 370, 401, 451, respectively. Any one of the filtercircuits 370, 401, 451 can be used to compensate for the compleximpedance load, while meeting passband performance requirements notattainable by the passive elliptical filter in the aforementioned IEEEarticle. More specifically, the filter circuit 370 brings the cutofffrequency down to 15KHz, in addition to raising the passband return lossto above 20 dB at 4KHz. This compares favorably with the 42.5KHz cutoffand 12 dB return loss at 4KHz for the 8th order modified elliptic lowpass filter in the IEEE article. Further, the characteristics (i.e.,frequency responses, passband return losses and stop band performances)of filter circuits 401, 451 are listed in Tables 3 and 4.

                  TABLE 3                                                         ______________________________________                                        Frequency Response and Return Loss                                            of the filters depicted in FIGS. 7 and 8                                                POTS to          Twisted Pair                                                 Twisted Pair     to POTS                                                      FIG. 7 FIG. 8        FIG. 7 FIG. 8                                            Filter Filter        Filter Filter                                            401    451           401    451                                     ______________________________________                                        Insertion                                                                            1000 Hz  1.0    dB  1.0  dB   1.0  dB  1.0  dB                         Loss                                                                          Frequency                                                                             200 Hz  0.9    dB  0.8  dB   0.8  dB  0.8  dB                         Response                                                                              500 Hz  0.5    dB  0.5  dB   0.5  dB  0.5  dB                         ("-"    800 Hz  0.2    dB  0.2  dB   0.2  dB  0.2  dB                         means loss                                                                           1000 Hz  0        0       0      0                                     compared        (reference)                                                                            (reference)                                                                           (reference)                                                                          (reference)                           to     1500 Hz  -0.4   dB  -0.4 dB   -0.4 dB  -0.4 dB                         1000 Hz)                                                                             2000 Hz  -0.6   dB  -0.6 dB   -0.6 dB  -0.6 dB                                2500 Hz  -0.8   dB  -0.7 dB   -0.8 dB  -0.7 dB                                3000 Hz  -0.8   dB  -0.7 dB   -0.8 dB  -0.7 dB                                3500 Hz  -0.7   dB  -0.7 dB   -0.7 dB  -0.7 dB                                4000 Hz  -0.6   dB  -0.6 dB   -0.6 dB  -0.6 dB                         Return  200 Hz  29     dB  29   dB   29   dB  29   dB                         Loss    500 Hz  24     dB  24   dB   24   dB  24   dB                                 800 Hz  22     dB  22   dB   23   dB  23   dB                                1000 Hz  22     dB  22   dB   23   dB  23   dB                                1500 Hz  23     dB  23   dB   24   dB  24   dB                                2000 Hz  24     dB  25   dB   26   dB  25   dB                                2500 Hz  25     dB  26   dB   25   dB  24   dB                                3000 Hz  25     dB  26   dB   24   dB  23   dB                                3500 Hz  24     dB  25   dB   24   dB  22   dB                                4000 Hz  22     dB  24   dB   23   dB  21   dB                         ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        The Downstream Stopband Performance                                           of the Filters depicted in FIGS. 7 and 8                                      Stopband Performance (POTS to Twisted Pair)                                             Insertion Loss (100 ohm source and term)                            Frequency   FIG. 7/Filter 401                                                                         FIG. 8/Filter 451                                     ______________________________________                                         20 KHz     12 dB       16 dB                                                  30 KHz     26 dB       24 dB                                                  40 Khz     68 dB       59 dB                                                  80 KHz     37 dB       34 dB                                                 100 KHz     39 dB       37 dB                                                 300 KHz     57 dB       57 dB                                                 500 KHz     67 dB       68 dB                                                 ______________________________________                                    

Unlike the dual-mode low-pass filter circuit 270 of FIG. 5, the filters370, 401, 451 do not change based upon the on-hook/off-hook signal.Nevertheless, filters 370, 401, 451 have acceptable passband andstopband performance for an ATU-R ADSL (i.e., remote unit). The stopbandperformance of this design is in the range of 40 dB-110 dB in the ADSLfrequency range. The remote ATU-R ADSL unit's signal recovery bandwidthstarts above 200KHz for the downstream data path. The stopbandattenuation of these filters at 200KHz is above the required minimum asshown in Tables 3 and 4. This proves to be an acceptable attenuation tosuppress errors caused by ring trip transients, dial pulsing, ringingand on/off hook transitions while maintaining excellent passbandcharacteristics. The attenuation at the upstream bandwidth starting at40KHz is about 40 dB. Since the ATU-R card generates the upstream data,the signal to noise ratio at the ATU-R card is excellent and the signalplus noise (generated by any signalling transients) is attenuated by thecable to the ATU-C. Therefore no upstream errors are generated by thetelephony signalling.

At the ATU-C (i.e., central unit), the upstream signal recovery rangestarts at 40KHz in the case of CAP modulation. However, the filter 370has an attenuation of about 40 dB at 40KHz. (The filters 401 and 451have attenuations of about 68 dB and 59 dB at 40KHz, respectively.)Thus, if one of the filters 370, 401 or 451 were applied at the ATU-C,it most likely would not have enough attenuation to suppress upstreamdata errors during application and removal of the central office ringrelay. However, as discussed above, the dual-mode filter 270 of FIG. 5could be used at the ATU-C, since the on-hook attenuation of thissplitter is 100 dB at 40KHz.

Thus, any one of the filters 370, 401, 451 can be used at the remoteunit as the low pass filter 240 shown in FIG. 2, while the filter 270 ofFIG. 5 can be used at the central unit as the low pass filter 230 ofFIG. 2.

Now describing FIG. 6 in detail, there are two main differences betweenthe two low-pass filter circuits 270 of FIG. 5 and 370 of FIG. 6. Unlikedual-mode filter 270, single-mode filter 370 does not require a relay,or equivalent, to selectively activate additional poles. In addition,filter 370 has an auxiliary inductor stage 390 between the passbandshaping circuit 372 and the inductor L14 which connects to the twistedpair.

The passband shaping circuit 372 comprises coupled inductor L12 andfirst and second pole circuits 374, 376, which are implemented as an RCnetwork. Inductor L12 comprises first and second coils 378, 380. Firstpole circuit 374 is connected in parallel between positive 378a andnegative 378b terminals of first coil 378 of inductor L12, while secondpole circuit 376 is connected in parallel between positive 380a andnegative 380b terminals of second coil 380 of inductor L12.

The first pole circuit 374 comprises resistors R24 and R25 connected inseries between terminals 378a, 378b, and a shunt capacitor C22 connectedacross resistor R25, between negative terminal 378b and a first node 382defined between R24 and R25. Similarly, the second pole circuit 376comprises resistors R23 and R22 connected in series between terminals380b, 380a, and a shunt capacitor C23 connected across resistor R22,between positive terminal 380a and a second node 384 defined between R23and R22.

Viewed from the POTS side, the passband shaping circuit 372 has a firstport 386 comprising the positive terminal 378a of L12's first coil 378,and the negative terminal 380b of L12's second coil 380. Viewed from thetwisted pair side, the passband shaping circuit 372 has a second port388 comprising the negative terminal 378b of L12's first coil 378, andthe positive terminal 380a of L12's second coil 380.

The auxiliary inductor stage 390 comprises coupled inductor L13 andcapacitors C27, C28. Capacitor C27 is connected in parallel betweenterminals of the first coil 392 of coupled inductor L13, while capacitorC28 is connected in parallel between terminals of the second coil 394 ofcoupled inductor L13. Capacitors C27 and C28 serve to hasten thetransition band between the passband and the stopband.

On the POTS side, the filter circuit 370 also comprises coupled inductorL11 connected to snubber circuitry comprising R26, D14, D15 and C26, notunlike the arrangement seen in FIG. 5. An isolation capacitor C21 isconnected between the positive terminal 378a of the first coil 378 ofL12 and the negative terminal 380b of the second coil 380 of L12. Table5 lists the values for the discrete elements used in the circuit 370 ofFIG. 6.

                  TABLE 5                                                         ______________________________________                                        Component Values for Filter Circuit of FIG. 6                                 COMPONENT      VALUE                                                          ______________________________________                                        L11                      5-50   mH                                            L12                      35     mH                                            L13                      3      mH                                            L14                      5      mH                                            R22                      50     Ω                                       R23                      35     Ω                                       R24                      35     Ω                                       R25                      50     Ω                                       R26                      50     KΩ                                      C21                      20     nf                                            C22                      1      μF                                         C23                      1      μF                                         C24                      12     nF                                            C26                      100    nF                                            C27                      12     nF                                            C28                      12     nF                                            ______________________________________                                    

With respect to FIG. 7, the filter 401 functions substantially identicalto the filter 370 of FIG. 6. Further, the filter circuit 401 includesdiscrete components having substantially identical values as thecorresponding values of the discrete components depicted in the circuit370 of FIG. 6. One obvious difference between filter 401 of FIG. 7 andfilter 370 of FIG. 6 is that filter 401 does not include a snubbercircuit. The snubber circuit is not included in order to reduce theoverall size and manufacturing cost of the filter 401. Table 6 lists thevalues for the discrete elements used in the filter circuit 401 of FIG.7.

                  TABLE 4                                                         ______________________________________                                        Componenet Values for Filter Circuit of FIG. 7                                COMPONENT      VALUE                                                          ______________________________________                                        L71                     2.5-50  mH                                            L72                     35      mH                                            L73                     3       mH                                            L74                     5       mH                                            R72                     50      Ω                                       R73                     35      Ω                                       R74                     35      Ω                                       R75                     50      Ω                                       CM7                     20      nF                                            C72                     1       μF                                         C73                     1       μF                                         C74                     12      nF                                            C75                     15      nF                                            C76                     15      nF                                            ______________________________________                                    

Now discussing FIG. 8 in detail, the filter 451 includes a passbandshaping circuit 472 that comprises coupled inductor L82 and first andsecond pole circuits 474, 476. The pole circuits 474, 480 areimplemented as RC networks. Inductor L82 comprises first and secondcoils 478, 480. First pole circuit 474 is connected in parallel betweenpositive 478a and negative 478b terminals of first coil 478 of inductorL82, while second pole circuit 476 is connected in parallel betweenpositive 480a negative 480b terminals of second coil 480 of inductorL82.

The first pole circuit 474 comprises resistors R84 and R85 connected inseries between terminals 478a, 478b, and a shunt capacitor C82 connectedacross resistor R85, between negative terminal 478b and a first node 482defined between R84 and R85. Similarly, the second pole circuit 476comprises resistors R83 and R82 connected in series between terminals480b, 480a, and a shunt capacitor C83 connected across resistor R82,between positive terminal 480a and a second node 484 defined between R83and R82.

Viewed from the POTS side, the passband shaping circuit 472 has a firstport 486 comprising the positive terminal 478a of L82's first coil 478,and the negative terminal 480b of L82's second coil 480. Viewed from thetwisted pair side, the passband shaping circuit 472 has a second port488 comprising the negative terminal 478b of L82's first coil 478, andthe positive terminal 480a of L82's second coil 480. As it has beenshown, the above described passband shaping circuit 472 is substantiallysimilar to the passband shaping circuit 372 of FIG. 6.

On the twisted pair side, the filter 451 includes a capacitor C84connected between the negative terminal 478b of the first coil 478 ofL82 and the positive terminal 480a of second coil 480 of L82. The filter451 then includes a coupled inductor L84 having first and second coils.One end of the first coil is connected to the negative terminal 478b ofthe first coil 478 of L82, while one end of the second coil is connectedto the positive terminal 480a of second coil 480 of L82. The inductorL84 acts as an open circuit viewed from the twisted pair side.

On the first port 486 side, an isolation capacitor CM8 is connectedbetween the positive terminal 478a of the first coil 478 of L82 and thenegative terminal 480b of the second coil 480 of L82. In addition, acommon mode choke inductor L81 is connected to the first port 486 sideof the pass band shaping circuit 472. As a common mode choke inductor,the inductor L81 can also be located between the capacitor C84 and theinductor L84 or between the inductor L84 and the twisted pair. It isunderstood that the common choke inductors of FIGS. 5-7 may also berepositioned in a similar manner.

The filter 451 does not include an auxiliary inductor stage between itspassband shaping circuit 472 and the inductor L84 which connects to thetwisted pair. By not including an auxiliary inductor stage, the overallsize and manufacturing cost of the filter 451 are reduced withoutharming performance characteristics thereof. It should also be notedthat the filter circuit 451 may also comprise snubber circuitry notunlike the arrangement shown in FIG. 5. The snubber circuitry is notincluded in the filter 451 of FIG. 8, again, in order to reduce theoverall size and manufacturing cost of the filter 451. Table 7 lists thevalues for the discrete elements used in the filter circuit 451 of FIG.8.

                  TABLE 7                                                         ______________________________________                                        Componenet Values for Filter Circuit of FIG. 8                                COMPONENT      VALUE                                                          ______________________________________                                        L81                     2.5-50  mH                                            L82                     35      mH                                            L84                     8       mH                                            R82                     50      Ω                                       R83                     35      Ω                                       R84                     35      Ω                                       R85                     50      Ω                                       CM8                     20      nF                                            C82                     1       μF                                         C83                     1       μF                                         C84                     12      nF                                            ______________________________________                                    

While the above invention has been described with reference to certainpreferred embodiments, it should be kept in mind that the scope of thepresent invention is not limited to these. For instance, the discreteelements listed in Tables 1-2 and 5-7 have 1-5% part tolerances.Further, the values of the discrete elements can vary up to 20% from thelisted values and still meet the filter characteristics described above.One skilled in the art may find variations of these preferredembodiments which, nevertheless, fall within the spirit of the presentinvention, whose scope is defined by the claims set forth below.

What is claimed is:
 1. A filter comprising:a first inductor having firstand second coils, each of said coils having a first and a secondterminal; first and second resistors connected in series between thefirst and second terminals of said first coil; a first capacitorconnected in parallel with the first resistor, said first capacitorconnected between one of the terminals of said first coil and a firstnode defined between said first and second resistors; third and fourthresistors connected in series between the first and second terminals ofsaid second coil; a second capacitor connected in parallel with thethird resistor, said second capacitor connected between one of theterminals of said second coil and a second node defined between saidthird and fourth resistors, wherein said first terminals of said firstand second coils define a first port of said filter, and said secondterminals of said first and second coils define a second port of saidfilter; and a third capacitor connected in parallel with said firstport.
 2. The filter of claim 1, wherein the first and third resistorshave substantially identical resistance, the second and fourth resistorshave substantially identical resistance, and the first and secondcapacitors have substantially identical capacitance.
 3. The filter ofclaim 1, further comprisinga fourth capacitor connected in parallel withsaid second port.
 4. The filter of claim 1, further comprising a secondinductor having a second pair of coils, each of said second pair ofcoils being connected to a different terminal of said second port. 5.The filter of claim 4, further comprising a third inductor having athird pair of coils, each of said second pair of coils being connectedto a different terminal of said first port.
 6. The filter of claim 5,further comprisinga fourth capacitor connected in parallel with saidsecond port.
 7. An asymmetric digital subscriber line (ADSL) systemconnecting a public switched telephone network (PSTN) to a plain oldtelephone system (POTS) switchable between an on-hook mode and anoff-hook mode, said ADSL system comprising:a central unit comprising afirst filter having a first connection to said PSTN; a remote unitcomprising a second filter having a first connection to said POTS; and atransmission line connecting said first and second filters, wherein saidsecond filter comprises:a first inductor having first and second coils,each of said coils having a first and a second terminal; first andsecond resistors connected in series between the first and secondterminals of said first coil, a first capacitor connected in parallelwith the first resistor, said first capacitor connected between one ofthe terminals of said first coil and a first node defined between saidfirst and second resistors; third and fourth resistors connected inseries between the first and second terminals of said second coil; asecond capacitor connected in parallel with the third resistor, saidsecond capacitor connected between one of the terminals of said secondcoil and a second node defined between said third and fourth resistors,wherein said first terminals of said first and second coils define afirst port of said second filter, and said second terminals of saidfirst and second coils define a second port of said second filter; and athird capacitor connected in parallel with said first port.
 8. Thesystem of claim 7, wherein the first and third resistors of the secondfilter have substantially identical resistance, the second and fourthresistors have substantially identical resistance, and the first andsecond capacitors have substantially capacitance.
 9. The system of claim7, wherein the second filter further comprises:a fourth capacitorconnected in parallel with said second port.
 10. The system of claim 7,wherein the second filter further comprisesa second inductor having asecond pair of coils, each of said second pair of coils being connectedto a different terminal of said second port.
 11. The system of claim 10,wherein the second filter further comprises a third inductor having athird pair of coils, each of said third pair of coils being connected toa different terminal of said first port.
 12. The filter of claim 11,further comprisinga fourth capacitor connected in parallel with saidsecond port.
 13. The system of claim 7, further comprisinga detectorarranged to output a first signal reflective of whether said POTS ison-hook or off-hook, wherein said first filter is arranged to operate ina first mode when said first signal indicates that said POTS is on-hookand in a second mode when said first signal indicates that said POTS isoff-hook.
 14. A filter comprising:a first inductor having first andsecond coils, each of said coils having a first and a second terminal; afirst Resistor-Capacitor (RC) network connected in parallel with saidfirst coil and being connected to said first and second terminals ofsaid first coil; a second RC network connected in parallel with saidsecond coil and being connected to said first and second terminals ofsaid second coil, said first and second RC networks being substantiallyidentical; and a first capacitor connected to the first terminals offirst and second coils.
 15. The filter of claim 14, further comprisingasecond capacitor connected to the second terminals of the first andsecond coils.
 16. The filter of claim 14, wherein each of said first andsecond RC networks comprise first and second resistors connected inseries between the first and second terminals of a corresponding coil,and a second capacitor connected in parallel with the first resistorbetween one of said terminals of that coil and a first node definedbetween the first and second resistors.
 17. The filter of claim 14,further comprisinga second inductor having a second pair of coils, eachof said second pair of coils being connected to a different terminal ofa first port of said filter, wherein said first terminals of said firstand second coils define said first port of said filter.
 18. The filterof claim 14, further comprisinga second inductor having a second pair ofcoils, each of said second pair of coils being connected to a differentterminal of a second port of said filter, wherein said second terminalsof said first and second coils define a second port of said filter. 19.The filter of claim 18, further comprisinga third inductor having athird pair of coils, each of said third pair of coils being connected toa different terminal of a first port of said filter, wherein said firstterminals of said first and second coils define the first port of saidfilter.
 20. The filter of claim 19, further comprisinga second capacitorconnected to the second terminals of the first and second coils.